The invention relates to a turbomachine disk comprising a peripheral slot having sidewalls and a loading window, and a plurality of blades retained in said peripheral slot, said blades comprising roots of the hammer head type adapted to be introduced into said slot through said loading window and to be held in said slot by collaboration of shape with said sidewalls, a radial clearance being provided between the bottom of said slot and the underside of said blade roots, in which there is provided a device for immobilizing said blades in said peripheral slot, said immobilizing device adapted to be introduced into said slot through said loading window and comprising a locking element and a radial manipulating screw having a head, said locking element adapted to be arranged in a space separating two adjacent blade roots and to be raised up, under the action of said manipulating screw, into a lock housing formed in said sidewalls, said screw head resting against the bottom of said slot.
Several locking devices of this type may be provided per stage. In general, the screw head, which is wider than the screw shaft, is housed in a recess formed in the bottom of the slot in line with the corresponding lock housing. Because the screw head is wider than the shaft, the screw is rendered captive. In current constructions, the locking element consists of a protrusion formed on the upper face of a body which, in a locking position of the device, rests against the sidewalls of the slot near the neck of the slot. This body has a lower base which, when the device is in the locking position, is spaced from the screw head so as to allow said body to slide in the slot when the blades are being mounted. Indeed, in order to allow this sliding, the protrusion formed on the body has to lie in the slot. The base of the body then rests against the screw head and lies near the bottom of the slot.
When the device is positioned facing the lock housing, by rotating all the blades about the disk, the body is raised up toward the outside under the action of the manipulating screw using a key that fits onto the opposite end of the screw to the head and is positioned in an orifice made in the platforms of the two adjacent blades. The protrusion is held in the lock housing by bracing between, on the one hand, the lock body resting in the neck of the disk and, on the other hand, the screw head housed in a recess formed in the bottom of the slot. The way the system works is dependent on the local deformation or by an attached thread or by any other means.
If the bracing effect is lost, only the self-locking effect retains the screw and prevents the protrusion from escaping from the lock housing.
By construction, the one-piece part consisting of the body and of its protrusion has no positive guide means as it slides in the slot during mounting. The screw head may therefore be incorrectly positioned in its recess during tightening, and this may result in subsequent movement of the screw head during operation of the turbomachine and a loss of the bracing effect. The tightening of an incorrectly positioned screw may also lead to seizure of the screw thread. Whereas this may hold the protrusion in the lock housing, this subsequently leads to difficulties in dismantling the device for maintenance operations.
Furthermore, during operation of the turbomachine, the screw is subjected to considerable centrifugal forces which, if the bracing effect is lost, may cause the screw to turn and therefore come out into the gas stream. This may, ultimately, release the protrusion from the lock housing when the turbomachine stops. from the lock housing when the turbomachine stops.
In another known immobilizing device, the locking element is mounted to slide axially in a radial opening of a body having a cross section tailored to the cross section of the slot and immobilized radially, and the screw head is trapped between the bottom of the slot and the base of the body. Means are provided for limiting the extent to which the locking element is raised up.
Thus, the body is guided as it slides in the slot, and this gives the axis of the screw a precise radial direction and avoids seizure. Furthermore, the screw head is radially immobilized with respect to the body, and the action of the centrifugal forces on the locking element, should the screw become partially slackened, prevents this screw from turning, because the screw head is then resting positively against the base of the body. The body is arranged between the roots of a pair of blades. It has a lower base situated above the bottom of the slot and a radial through-opening of noncircular cross section in which the locking element is slideably mounted under the action of the manipulating screw. The screw head is dimensioned to remain trapped between the bottom of the slot and the base of the body.
The cross section of the body in a plane perpendicular to the axis of the manipulating screw is greater than the cross section of the locking element in the same plane, and the cross section of the screw head is also greater than the cross section of the locking element so that the upper face of the screw head is adapted to bear against the base of the body. This technology is not suited to certain turbine disks because there is not enough room between the roots of two consecutive blades.
The object of the invention is to propose an immobilizing device which overcomes these disadvantages and which can be housed in a small circumferential space.
This object is achieved through the fact that said screw head is restrained in the radially outward direction by the two adjacent blade roots.
Thus, should the screw become slack during operation, under the action of centrifugal force, the outward between the bottom of the slot and the underside of the blade roots. When the screw head is resting positively against the blade roots, the same centrifugal forces acting on the locking element push the latter outward, and this prevents greater tightening of the screw and might even tend to cause it to turn in the opposite direction.
According to a simplified first embodiment, the diameter of the screw head is greater than the separation between said two adjacent blade roots. The immobilizing device therefore comprises two parts: the locking element and the manipulating screw.
According to a second embodiment of the invention, there is further provided an intermediate plate between said screw head and said locking element, the ends of said plate being arranged under said two adjacent blade roots.
According to another advantageous feature of the invention, said plate comprises means for preventing it from turning with respect to said locking element. These means consist, for example, of radial tabs which allow the locking element to be raised up during mounting.
Advantageously, the plate also comprises means for preventing the screw head from turning. These means preferably consist of radial tabs resting on flats of the screw head. These tabs are flexible so as to allow the screw to turn, during mounting, as the locking element is raised up.
According to another feature of the invention, the locking element has an upper protrusion or sleeve near the end of the screw which can be housed in an orifice made in the platforms of said two adjacent blades. This sleeve acts as a visual indicator that the locking indicator has been raised up.